Grid-connected inverters are used in energy supply systems, for example photovoltaic systems and wind energy installations. In grid-connected inverters, a voltage or current profile output at the output of the inverter follows the corresponding profile of the electric power grid. In the energy supply systems, generators, for example photovoltaic modules connected in series and/or parallel, generate a DC voltage, which is supplied to a DC voltage intermediate circuit, possibly after a change in voltage by a step-up converter. Direct current from the DC voltage intermediate circuit is converted into an alternating current that is suitable in terms of its frequency and voltage for feeding into the electric power grid by the inverter. This conversion can take place into a single-phase or multiphase alternating current, in particular three-phase alternating current. In this case, the inverter comprises an output bridge circuit which comprises one or more switching bridges which are generally equipped with power semiconductor switches, depending on the number of phases of the electric power grid into which it is intended to be fed in.
In this case, the power semiconductor switches are actuated in accordance with a specific modulation pattern in such a way that, in combination with filters which are arranged between the inverter and the electric power grid, an output current is generated which is as sinusoidal as possible. In pulse width modulation methods (PWM), which are often used, the power semiconductor switches are switched on and off with a switching frequency that is considerably higher than the frequency of the AC voltage in the electric power grid (for example a switching frequency of 3 to 30 kHz in comparison with a mains frequency of 50 or 60 Hz). Over the course of a period of the system frequency, in this case the ratio between the switch-on time and the switch-off time within a switching frequency period, referred to as the duty factor, is changed in such a way that a profile of the output current as sinusoidal as possible is provided. Known configurations for determining the duty factors or the switching times are, for example, the “sine-triangle modulation method”, the “space vector modulation method” (SVM) or modified sine-triangle modulation methods, for example the so-called “third-harmonic sine-triangle modulation method”. In these PWM methods, a periodic auxiliary signal, for example a triangular signal in the “sine-triangle modulation method” or a clock signal in the “SVM method” is used for determining the switching times.
Even in the case of relatively complex modulation methods, however, the AC voltage generated is typically not a purely sinusoidal signal, but shows, for example, frequency components at the switching frequency of the modulation method, which are referred to as voltage ripples.
In order to achieve relatively high output currents or powers, in particular in the case of relatively large photovoltaic systems, often two or more inverters are used in parallel. When these inverters are not completely isolated from one another on the output voltage side, compensation currents can occur between the inverters, which result in undesirable additional current loading for the power semiconductor switches of the output bridge circuit of the inverters. Such output currents occur, for example, when inverters are connected to one another both on the DC voltage side via a common intermediate circuit and on the AC voltage side, for example since they are connected directly to the electric power grid without a transformer interposed. On the DC voltage side, coupling can also be provided when no common intermediate circuit is used, but the modules or strings (a series connection of modules) are each grounded with one connection. On the AC voltage side, problematic mutual influencing of two inverters can also be provided via an inductive coupling. In a case of inductive coupling of two inverters, no DC compensation currents flow, but compensation currents with AC components at relatively high frequencies may flow, in particular due to the above-described voltage ripples which are not entirely avoidable during the inversion. Such a coupling can also be observed in installations in which transformers are arranged between the inverters and the electric power grid.
The document DE 10 2008 056 256 A1 shows an inverter arrangement having a plurality of inverters connected in parallel, in which compensation currents between the inverters can be avoided by virtue of the fact that corresponding power semiconductor switches of the individual inverters are actuated at the same time. This is achieved in that one of the inverters, referred to as the master inverter, generates actuation signals for the power semiconductors which are transmitted via corresponding lines to each of the further inverters, which are referred to as slave inverters. However, this method is only practicable in the case of inverters that are arranged physically close to one another.
Document US 2008/0265680 A1 describes an arrangement of a plurality of inverters that are coupled directly with their outputs. The inverters are controlled by PWM methods, wherein the auxiliary signals used in the process are synchronized using a grid voltage. As a result, transmission of actuation signals for the output bridge arrangement of the inverters is not necessary. The electric power grid is used as a synchronization connection. The method is well suitable for directly interconnected inverters. In the case of inverters that are coupled inductively, for example via transformers, however, it has been demonstrated that compensation currents are not completely suppressed.